This invention pertains to an implement suitable for the precise violation of vertebral body end plates during the performance of interbody spinal fusion techniques.
Of all animals possessing a backbone, human beings are the only creatures who remain upright for significant periods of time. From an evolutionary standpoint, this erect posture has conferred a number of strategic benefits, not the least of which is freeing the upper limbs for purposes other than locomotion. From an anthropologic standpoint, it is also evident that this unique evolutionary adaption is a relatively recent change and as such has not benefitted from natural selection as much as have backbones held in the horizontal attitude. As a result, the stresses acting upon the human backbone (or “vertebral column”), are unique in many senses, and result in a variety of problems or disease states that are peculiar to the human species.
The human vertebral column is essentially a tower of bones held upright by fibrous bands called ligaments and contractile elements called muscles. There are seven bones in the neck or cervical region, twelve in the chest or thoracic region, and five in the low back or lumbar region. There are also five bones in the pelvis or sacral region which are normally fused together and form the back part of the pelvis. This column of bones is critical for protecting the delicate spinal cord and nerves, and for providing structural support for the entire body.
Between the vertebral bones themselves exist soft tissue structures—discs—composed of fibrous tissue and cartilage which are compressible and act as shock absorbers for sudden downward forces on the upright column. More importantly, the discs allow the bones to move independently of each other to permit functional mobility of the column of spinal vertebrae. Unfortunately, the repetitive forces which act on these intervertebral discs during repetitive day-to-day activities of bending, lifting and twisting cause them to break down or degenerate over time.
Presumably because of humans' upright posture, their intervertebral discs have a high propensity to degenerate. Overt trauma, or covert trauma occurring in the course of repetitive activities disproportionately affect more highly mobile areas of the spine. Disruption of a favorable for bony fusion to occur, the removal of end plate actually weakens the vertebral body making it more likely for the prosthesis itself to sink or subside into the adjacent soft cancellous bone of the adjacent vertebral bodies. If subsidence occurs, the stabilization provided by the prosthesis as regards adjacent vertebral elements is lost, and non-union can occur. Non-union or failure of the vertebral elements to fuse is obviously undesirable and in many instances leads to failure of the surgical procedure and a requirement for additional surgery.
While violation of the end plate is necessary to expose the inner cancellous bone to facilitate fusion, it is desirable to violate as little end plate as possible so that the remaining end plate can continue to provide structural support for the vertebral body thereby reducing or eliminating the chance of subsidence of the intervertebral prosthesis into the vertebral body itself.
It would be most desirable to disrupt end plate only over the portion of the prosthesis where vertebral cancerous bone would be exposed to bone graft material sequestered within the confines of the intervertebral prosthesis. This would allow the remaining intervening end plate to provide structural support to the vertebral body and provide a ready barrier to prevent prosthesis subsidence into the soft central cancellous bone of the vertebral body. Most present-day methods of implanting intervertebral prostheses require significant end plate disruption over the majority of the surface area of the prosthesis; subsidence of the implant not infrequently occurs.
To counteract this problem, it is desirable to disrupt or violate the vertebral body end plate exactly and exclusively at the apertures in the intervertebral prosthesis where cancerous bone and graft contact are to occur, thereby preserving end plate at all other areas of contact with the prosthesis. To achieve this idea of greater end plate support, precise end plate disruption limited only to the apertures allowing bone to graft contact must occur.